Study On Photoprotective Mechanisms Under Salt Stress In Kosteletzkya Virginica(L.) Presl

After different salt stress, a combined measurement of gas exchange and chlorophyll fluorescence was conducted to evaluate the effects of salt stress to photoinhibition, as well as the role of thermal dissipation, photorespiration and metabolism of active oxygen in photoprotective mechanisms of Kosteletzkya virginica(L.) Presl. The results are as follows:1. Effect of salt stress on different photoinhibition degrees of Kosteletzkya virginica seedlingsUnder different salt stress, different photoinhibition degrees formed in Kosteletzkya virginica. Maximal fluorescence (Fm), maximal photochemical efficiency of PSII (Fv/Fm), minimal fluorescence (Fo) increased and then decreased slightly. The efficiency of photosystemII photochemistry (ΦPSII), electron transport rate (ETR) decreased significantly. The results showed that salt stress aggravated photoinhibition of photosynthesis, but its photosynthetic reaction center hadn't been destroyed under salt stress conditions.2. Effect of salt stress on the growth of Kosteletzkya virginica seedlingsThe growth indexes-fresh weight and dry weight of Kosteletzkya virginica decreased with the increase of NaCl concentrations. However, the growth idexes of Kosteletzkya virginica treated with 100 mmol/L NaCl decreased slightly compared with the control, and then decreased significantly with the increase of NaCl concentration (above 200 mmol/L NaCl). These indicate that Kosteletzkya virginica can tolerance low concentration of salt stress, when the environment up to a certain concentration of salt until it inhibited the growth.3. Effect of salt stress on the photosynthetic characters of Kosteletzkya virginica seedlingsUnder the treatment with 100 mmol/L NaCl, Pn of Kosteletzkya virginica slightly increased compared with the control (P>0.05), but decreased significantly with the increase of NaCl concentration (P<0.01). With Pn decreasing, Gs and Ci decreased gradually while changes of Ls presented an opposite trend. It is obvious that at higher concentration of NaCl (200, 300 mmol/L NaCl) the decrease of Pn closely correlate with stomatal limitation factors.CO₂ compensation point and light compensation point increased significantly with the increase of NaCl concentration. It indicates that abilities of using light and CO2 decreased under the salt treatment. But decrease of light saturation densities with the increase of NaCl concentration indicates that photoinhibition is more likely to occur in strong light.AQY and Pn exhibited the similar trend under salt stress. The decrease of AQY indicates that the photoinhibition has happened.Chlorophyll a/b increased significantly with the increase of salt concentration up to 100 mmol/L and then decreased significantly. This indicates that thylakoids still has higher stack extent possibly under the low salt treatment.Under salt stress, many photosynthetic parameters, such as light compensation point, AQY, CO2 compensation point, light saturation densities and chlorophyll a/b indicate that salt stress aggravates photoinhibition of photosynthesis, but low concentrations of NaCl only has slightly effect on Kosteletzkya virginica. Therefore, Kosteletzkya virginica can adapt to the saline environment.4. The role of thermal dissipation in photoprotective mechanisms under salt stress of Kosteletzkya virginica seedlingsqNp measures a change in efficiency of thermal dissipation. qNp increased compared with the control under the treatment with 200, 300 mmol/L NaCl. It indicates that thermal dissipation of Kosteletzkya virginica significantly increased. The experiment showed that the thermal dissipation increased significantly with the decrease of the efficiency of photochemistry, such asΦPSII and qP. But it really made a positive contribution to protect the photosynthesis of Kosteletzkya virginica.4.1 The role ofâ–³pH-dependent quenching (qE) in photoprotective mechanisms under salt stress of Kosteletzkya virginica seedlingsqE is the major composition of qNp in Kosteletzkya virginica, and comprises 70% of qNp. In this experiment, qE and qNp exhibited the similar trend, which decreased slightly and then increased significantly with the increase of salt concentration. This indicates that under salt stress, qE that is dependent on the energization of the thylakoid membrane maybe an efficient means to dissipate the heat.4.2 The role of xanthophyll cycle in photoprotective mechanisms under salt stress of Kosteletzkya virginica seedlingsThe de-epoxidation content of xanthophyll cycle (DPS) had the same trend with qE, which decreased slightly and then increased significantly with the increase of salt concentration. It is obvious that a stable and highâ–³pH resulting in the de-epoxidation of violaxanthin to form antheraxanthin and zeaxanthin, while the subsequent successfully excitate excess energy at saturating light of Kosteletzkya virginica.4.3 The role of state transition (qT) in photoprotective mechanisms under salt stress of Kosteletzkya virginica seedlings qT slightly increased under the treatment with 100 mmol/L NaCl and then decreased with the increase of NaCl concentration. This indicates that qT only makes a small contribution at low salt concentrations.4.4 The role of photoinhibition (qI) in photoprotective mechanisms under salt stressof Kosteletzkya virginica seedlings qI that is one of the composition of qNp exhibited only 20% of qNp. In this experiment, qI and DPS didn't exhibite the similar trend, only slightly increased with the increase of NaCl concentration. The experiment indicates that excepting of relating to the xanthophyll cycle, the slow relaxing quenching seems to refer to other two hypotheses.5. The role of photorespiration in photoprotective mechanisms under salt stress of Kosteletzkya virginica seedlingsPhotorespiration rate and GO activity exhibited the similar trend, which decreased significantly with the increase of salt concentration. The experiment indicates that photorespiration isn't an efficient means to dissipate excessive light energy. The decreasing of photorespiration results in the energy dissipation switching to other photoprotective mechanisms.6. The role of reactive oxygen species in photoprotective mechanisms under salt stress of Kosteletzkya virginica seedlingsThe results showed that salt stress aggravated the damage of membrane, while the activities of SOD and APX in leaves increased obviously as compared to the controlled plants. The contents of AsA, GSH and the relative contents of Car, Anth in leaves increased distinctly. Under different salt concentrations of NaCl, the activity of CAT showed no obviously change, only increased slightly. It indicates that the reactive oxygen species in Kosteletzkya virginica is an important protection mechanism under different salt concentrations.It is obvious that salt stress aggravated photoinhibition of photosynthesis, which in turn started up several photoprotective mechanisms in Kosteletzkya virginica. Thereinto, thermal dissipation based on light-harvesting antenna and reactive oxygen species showed obviously change in higher salt concentration. It seems that these two forms of mechanisms are very important to Kosteletzkya virginica under higher salt stress.